Coaxial horns with cross-polarized feeds of different frequencies



April 21, 1970 Filed may 5. 1967 D. G. WARE ET AL COAXIAL HORNS WITHCROSS-POLARIZED FEEDS OF DIFFERENT FREQUENCIES 2 Sheets-Sheet iInventors DAV/O G. WARE GRAHAM L-AMr/IOLOflN By gf A gen f D. G. WARE ETAL COAXIAL HORNS WITH GROSS-POLARIZED FEEDS April 2 1, 1970 OF DIFFERENTFREQUENCIES 2 Sheets-Sheet 2 Filed May 5, 1967 70 HOE/V6 yaw Agent I. On Mumm n R I A 8 DMV. wAB AM on .G

United States Patent O 3,508,277 COAXIAL HORNS WITH CROSS-POLARIZEDFEEDS OF DIFFERENT FREQUENCIES David George Ware, Bexley, and GrahamLeslie Bartholomew, London, England, assignors to International StandardElectric Corporation, New York, N.Y., a corporation of Delaware FiledMay 5, 1967, Ser. No. 636,440 Claims priority, application GreatBritain, May 27, 1966, 23,912/ 66 Int. Cl. H01q 13/00 US. Cl. 343-776 17Claims ABSTRACT OF THE DISCLOSURE A waveguide junction for combiningfour independent microwave signals and feeding them to a common load.Two circular Waveguides are mounted coaxially with respect to each otherand two signals lying in an upper frequency band and having orthogonalplanes of polarization are fed into the inner waveguide. Two modeconverters are coupled to the outer waveguide to convert two respectiveinput signals lying in a lower frequency band into two H coaxial modesignals having orthogonal planes of polarization. The two lowerfrequency band signals are transmitted between the two waveguides. Hornsare provided at the ends of the waveguides for feeding to a common load,such as an antenna.

BACKGROUND OF THE INVENTION This invention relates to waveguidejunctions, and more particularly to a waveguide junction for combining aplurality of independent microwave signals and feeding them to a commonload, such as a microwave antenna.

Microwave antennae and towers to support them are generally large andexpensive structures. It is therefore desirable to reduce the number ofantennae on a given radio link route to a minimum. One method reducingthe number of antennae on a given radio link is to use a single antennato radiate or receive two or more microwave signals occupying separatefrequency bands. This type of system is well known in the art.Furthermore, a parabolic reflector can be used which can accommodatesignals lying in a given frequency band, but having mutuallyperpendicular planes of polarization.

Therefore, the main object of this invention is to provide a waveguidejunction for combining more than two independent microwave signals andfor feeding them to a common load, such as a microwave antenna.

SUMMARY OF THE INVENTION According to the invention there is provided awaveguide junction for the transmission of up to four multiplexmicrowave signals, two of said signals lying in an upper frequency bandand two of said signals lying in a lower frequency band, the two signalsin each of said bands having mutually perpendicular planes ofpolarization. The junction comprises an inner circular waveguide fortransmission of the signals in the upper frequency band and an outercircular waveguide for transmission of the signals in the lowerfrequency band, the two waveguides forming a coaxial structure.Apparatus is coupled to the outer waveguide for converting the lowerfrequency signals into respective H coaxial mode signals having 1mutually perpendicular planes of polarization and for launching said Hcoaxial mode signals within the outer circular waveguide.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a partly sectionedperspective view of the junction according to the invention;

3,508,277 Patented Apr. 21, 1970 FIGS. 2 and 3 are transverse and axialsections through one of the mode converters of the junctions; and

FIG. 4 is a diagrammatic view of a clamp; and

FIG. 5 is a partly sectioned perspective view of a portion of thejunction of FIG. 1 which is modified according to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIG. 1, a centralwaveguide 1 of circular cross section is located inside an outerwaveguide 2, also of circular cross section, and is axially aligned withthe latter. The outer waveguide 2 has a non-reflecting termination 3 atone end thereof.

Two rectangular waveguides, 4 and 5, having their larger dimensionparallel to the longitudinal axis of the coaxial structure are joined tothe outer waveguide 2. The longitudinal axes of these rectangularwaveguides 4 and 5 are normal to each other and also to the common axisofthe coaxial waveguide structure. At each junction of one of therectangular waveguides with waveguide 2, the wall of the outer waveguide2 is provided with a rectangular aperture 6 having dimensions smallerthan the corresponding dimensions of the broad and narrow walls of theWaveguides 4 and 5. These apertures 6 form resonant irises in therectangular waveguides. The aperture for waveguide 5 is not shown in thedrawings for the sake of clarity and it is pointed out here that it issimilar to the aperture shown for waveguide 4.

Associated with each junction of the rectangular waveguides 4 and 5 andthe outer circular waveguide 2 are two conducting planes or septa 7, 8,9 and 10, of which only 7 and 9 are visible in FIG. 1. Septum 8 isvisible in FIG. 2. The two septa associated with a particularrectangular waveguide interconnect the outer and inner circularwaveguides, both said septa lying in a plane which includes the axis ofthe inner Waveguide, but is normal to the axis of the associatedrectangular waveguide. The exact axial position of the septa isdetermined by experiment, but in general the narrow edges of the septanearest to the horns '11 and 12 will approximately coincide with anarrow edge of the resonant iris which is furthest from the horns 1.1and 12 at the right end of the coaxial waveguides. The approximateposition of the septa relative to the resonant iris of a mode converteris indicated by the dashed line 13 in FIG. 3. The axial length of eachseptum is substantially equal to one half wavelength of the meanfrequency of the signals lying in the lower band.

The inner and outer circular wave-guides, 1 and 2, respectively,terminate in horns 11 and 12 which. radiate energy from the circularwaveguides to a microwave aerial, not shown. The inner and outerwaveguides 1 and 2 near the horns are kept in coaxial alignment by aseparator 13 made of insulating material.

The operation of the junction will now be described.

Two signals, representing two respective communication channels, lyingin an upper frequency band are transmitted through the central, circularWaveguide 1. Each of these signals is propagated as a H circularlypolarized wave, but the planes of polarization of these waves are normalto each other. These-two waves are derived from two separate H waves bywell known mode conversion means, not shown, which are connected to .thecentral waveguide 1 at the end opposite to the born 12.

Two signals, representing two other respective communication channels,occupy a lower frequency band and are transmitted as H coaxial modewaves in the annular space between the two waveguides 1 and 2. Theseparation of these second two waves is based on the use of waves havingplanes of polarization which are normal to each other. These two wavesare derived from two H waves transmitted in rectangular waveguides (notshown) which are connected to rectangular waveguides 4 and 5. Theorientation of these not shown waveguides is such that the direction ofthe vector representing the electric field is normal to the axis of thecircular waveguides 1 and 2. The resonant irises at the junctions of therectangular waveguides 4 and 5 and outer circular waveguide 2 compensatefor any impedance irregularities introduced by .the junction and reducereflections. To correct for residual discontinuities, capacitive trimmerscrews 14 are provided on both rectangular waveguides. For clarity, onlythe tirmmer screws 14 associated with waveguide 4 are shown in FIG. 1.If required additional screws may be used on the circular waveguide, theaxis of these screws lying in the plane of the E vector.

The H coaxial mode wave launched in the outer circular waveguide 2 wouldnormally propagate in both directions. Since propagation is desired onlyin the direction towards the horns 11 and 12, two metal septa 7, 8 and9, are placed in the space between the two circular waveguides 1 and 2and behind the wall aperture 6 forming the resonant iris. The plane inwhich the septa 7, 8 and 9, 10 lie is parallel to the direction of theelectrical field of the signal. The septa are in electrical contact withboth circular waveguides 1 and 2 and present an effective short circuitto the H coaxial mode wave propagating to the left. See FIGS. 2 and 3for more detailed cross-sectional views of the construction of a typicalWaveguide junction in the vicinity of rectangular waveguide 4. Foroptimum performance the axial length of the septa is chosen to beapproximately equal to M2 of the wave corresponding to the meanfrequency of the signal. The wave is .thus reflected by the septatowards the horns 11 and 12 and the propagation of the wave in theopposite direction is prevented.

The second wave in the lower frequency band is launched in a similarmanner as the first wave in the lower frequency band, except that therectangular waveguide, the iris and the septa are circumferentiallydisplaced by 90 relative to the first mode converter. It should be notedthat this H coaxial mode wave propagates past the septa of the firstconverter substantially without attenuation, since the plane of thesepta of the first converter is normal to the direction of the electricfield of the wave launched by the second converter.

The axial separation of the two rectangular waveguides 4 and 5 is notcritical, but interaction between the two waves in the lower frequencyband is reduced if this separation is not less than a wavelength of thesignal.

In order to reduce or eliminate distortions which might arise due toreflections of waves propagated in the outer waveguide in the unwanteddirection past the septa, a nonreflecting termination 3 is provided toabsorb these unwanted Waves.

In the mode converters described with reference to FIG. 1 the directionof the electrical vectors in each of the rectangular waveguides 4 and 5was normal to the axis of the circular waveguide. It is however, alsopossible to achieve the same results if each of the rectangularwaveguides 5 and 6 and the resonant irises are turned round their axesthrough 90, so that the direction of the electrical vector is parallelto the axis of the circular Waveguide. In this case however the planecontaining each pair of septa is also rotated round the axis of thecircuclar waveguide through 90.

In order to improve the coupling between the coaxial waveguides and themicrowave antenna used for radiating or receiving the signals, thecircular waveguides 1 and 2 are terminated in horns 11 and 12, which aredesigned by known methods to give the desired pattern of radiation andimpedance match. In order to provide rigidity, and to maintain thecoaxial relationship of waveguides 1 and 2, the two coaxial horns 11 and12 are separated by an insert 13 made of insulating material having lowdielectric losses. The axial position of separator 13 is chosen tosatisfy two separate requirements.

The first of these requirements is that the reflection caused byseparator 13 should substantially reduce the reflection caused by theimpedance discontinuity at the mouth of the horn. The optimum axiallength and position of separator 13 are therefore determined byexperiment. The second requirement for the separator 13 is that while itmust transmit freely waves lying in the lower frequency band, it mustreflect waves lying in the upper frequency band. This requirement arisesfrom the need to reduce crosstalk between channels radiated by the innerhorn into the outer horns. For this reason the separator 13 must have atransmission characteristic corresponding to a lowpass filter. This typeof characteristic is obtained by placing a number of dielectricobstacles 19, 20 and 21 (see FIG. 5) in the space between the waveguides1 and 2, the separation between two adjacent obstacles beingsubstantially equal to one quarter wavelength at the frequency of thesignal to be passed, and half a wavelength of the frequency of thesignal to be stopped. The separator 13 as shown in FIG. 1 will thereforein practice he replaced by a plurality of insulating washers 19, 20, 21as illustrated in FIG. 5. Of course it is possible to simplify thedesign by separating the filter and reflection compensating functionsreferred to above. In this case the disc shaped obstacles 19, 20 and 21would only perform the filter function, the reflection compensatingfunction being performed by other and independent means, for exampleobstacles made of conducting material.

A dielectric obstacle 22 may also be placed in the central waveguide 1in the vicinity of born 12. In this case, however, its sole function isto compensate for unwanted reflections generated at the mouth of horn12. As will become apparent later, the filter function of the obstacle22 in this case is not required. The obstacles in both waveguides 1 and2 perform a further important function of sealing the overall waveguidestructure against atmospheric influences.

If the minimum and maximum frequencies defining the lower frequency bandare designated by and f and the minimum and maximum frequencies definingthe upper frequency band are designated by f and respectively, then thefollowing considerations apply for the choice of diameters of the innerand outer circular waveguides, 1 and 2, respectively. The inner diameterd of the inner waveguide 1 is chosen so that signals lying in the upperfrequency band will propagate, but that signals lying in the lowerfrequency band cannot be propagated in it. In practice d would be chosento satisfy the inequality:

where A is the wavelength at frequency f and A is the wavelength atfrequency f Let al be the external diameter of the inner waveguide 1 andd be the internal diameter of the outer waveguide 2. d and d would, inpractice, be chosen to permit propagation in the H mode between thefrequencies i and f and to suppress propagation of all other waveguidemodes. Therefore d and d are chosen to satisfy the inequality:

where A is the wavelength at frequency f and A is the wavelength atfrequency f,,.

When imperfect circular waveguides are used to transmit waves in twoorthogonal polarizations some conversion of signals of one polarizationinto the other will take place. Such signal conversion should be reducedto a minimum since it will introduce distortion into the signalchannels. The most serious defect met with in circular waveguides istheir departure from true circularity due to various causes. Thesegeometrical distortions may vary in magnitude and direction along thelength of the waveguide and it is impossible to correct themindividually.

In the present junction, therefore, polarization crosstalk iscompensated for by providing distorting means to introduce deliberategeometrical distortion at one point along the axis of at least one ofthe circular waveguides. The purpose of the distorting means is to causea signal to be generated within the waveguide which has a magnitude andphase angle opposite to that of the spurious signals generated in thewaveguide due to the inherent geometrical distortions of the waveguide.The magnitude of the introduced geometric distortion and thecircumferential position of the distorting means determines themagnitride and phase of the signal which is being introduced to cancelout the spurious signals generated due to irregularities in thewaveguide.

Any arrangement which will exert a compressive force acting along adiameter of the waveguide and will cause the cross section of thewaveguide to become elliptical willbe satisfactory provided that themagnitude and direction of the deformation can be adjusted as required.A simple clamp meeting these requirements is shown in FIG. 4. Itcomprises two semicircular clamps 15 and 16 each having a radius ofcurvature slightly larger than the radius of the waveguide 2 with whichit is to be used. The two clamps 15 and 16 are pressed against thewaveguide by means of screws 17 and 18. The clamp of FIG. 4 may also beprovided for waveguide 1, but that this may easily be designed by oneordinarily skilled in the art within the spirit of this invention and istherefore not illustrated or described in detail herein.

The four independent channels provided by the junction can be usedeither for transmission or for reception. In general however they willbe used to provide two bidirectiorial communication channels.

It is to be understood that the foregoing description of specificexamples of this invention is made by way of example only and is not tobe considered as a limitation on its scope.

What we claim is:

1. A waveguide junction for transmitting a plurality of microwavesignals, two of said signals lying in an upper frequency band and two ofsaid signals lying in a lower frequency band, the two signals in each ofsaid bands having mutually perpendicular planes of polarization, saidjunction comprising:

an inner circular waveguide for transmission of said signals in saidupper frequency band;

an outer circular waveguide for transmission of said signals in saidlower frequency band;

means for mounting said inner waveguide coaxially within said outerwaveguide;

means for applying signals in said upper frequency band to said innercircular waveguide;

means for applying said signals in said lower frequency band to saidwaveguide junction including means coupled to said outer waveguide forconverting said lower frequency signals into respective H coaxial modesignals having mutually perpendicular planes of polarization; and

filter means coupled between said outer and inner waveguides fortransmitting signals lying in said lower frequency band annd forsubstantially reflecting signals lying in said upper frequency band.

2. A waveguide junction according to claim 1 wherein said means forapplying said lower frequency signals includes first and second modeconverting means coupled to said outer waveguide, each said modeconverting means being adapted to convert respective H coaxial modemicrowave signals in rectangular waveguides into respectice H coaxialmode signals having mutually perpendicular planes of polarization.

3. A waveguide junction according to claim 1 wherein said inner andouter waveguides each terminate in a horn at one end thereof forradiating said signals lying in said upper and lower frequency bands.

4. A waveguide junction according to claim 3 further comprising anonreflecting termination coupled to said outer waveguide at the endthereof opposite to said horn.

5. A waveguide junction according to claim 1 further comprising meanscoupled to said waveguide to compensate for undesired signals having aspurious plane of polarization within said circular waveguides.

6. A waveguide junction according to claim 2 wherein said first andSecond mode converting means includes first and second rectangularwaveguide sections, respectively, coupled to said outer waveguide.

7. A waveguide junction according to claim 6 wherein the respective axesof each of said two rectangular waveguide sections are normal to thelongitudinal axis of said circular waveguides and wherein saidrectangular waveguide sections have a circumferential separation of onsaid outerwaveguide.

8. A waveguide junction according to claim 6 further comprising:

a first resonant iris formed by a first aperture in the wall of saidouter waveguide for matching the impedance of said first rectangularwaveguide to said outer circular waveguide; and

a second resonant iris formed by a second aperture in the wall of saidouter waveguide for matching the impedance of said second rectangularwaveguide to said outer circular waveguide.

9. A waveguide junction according to claim 1 further comprising meanscoupled to said waveguides for causing each of said two H coaxial modesignals to be transmitted substantially in only one direction withinsaid outer waveguide.

10. A waveguide junction according to claim 9, wherein said means forcausing said signals to be transmitted substantially in only onedirection includes:

a first conducting septum coupled to said waveguides and extending in aplane parallel to the plane of the vector of the electric field of oneof said lower frequency signals; and

a second conducting septum coupled to said waveguides and extending in aplane parallel to the plane of the vector of the electric field of theother of said lower frequency signals.

11. A waveguide junction according to claim 10 wherein the axial lengthsof said first septum are substantially equal to one half the wavelengthof the mean frequency of one of said lower frequency signals and whereinthe axial lengths of said second septum are substantially equal to onehalf the wavelength of the mean frequency of the other of said lowerfrequency signals.

12. A waveguide junction according to claim 1 wherein the inner diameterof said inner waveguide is dimensioned so that the cut-off frequency ofsaid inner waveguide lies above the upper frequency of said lowerfrequency band.

13. A waveguide junction according to claim 1 wherein said filter meanscomprises a plurality of dielectric washers mounted between said innerand outer circular waveguides, said washers having an axial separationfrom each other substantially equal to one half the wavelength at themidband frequency of the upper frequency band, said filter furtherproviding a mechanical support between the inner and outer circularwaveguides.

14. A waveguide junction according to claim 1 wherein an obstacle ismounted within each of said two circular waveguides adjacent to saidhorns and in a position such that reflections caused by said obstaclesubstantially cancel reflections caused by the impedance discontinuitiesat the mouths of said horns.

15. A waveguide junction according to claim 5 wherein said compensatingmeans comprises:

means coupled to at least one of said circular waveguides for distortingthe circumferential shape thereof; and

means for adjusting said distorting means to vary the magnitude andangular position of the distortion provided by said distorting means;said distorting means causing a signal to be generated which willsubstantially cancel any spurious signals generated within saidwaveguides due to the inherent deviations from true circularity of saidwaveguides. 16. A waveguide junction according to claim 14 wherein eachsaid distorting means includes:

first and second arcuate portions having respective radii of curvaturelarger than that of its associated circular waveguide; and means forcoupling said first and second arcuate portions together and around theperiphery of one of said waveguides. 17. A Waveguide junction accordingto claim 3 further comprising a dielectric means mounted between saidfirst 8 and second waveguides in the vicinity of said horns formaintaining the coaxial relationship between said waveguides.

References Cited UNITED STATES PATENTS ELI LIEBERMAN, Primary ExaminerUS. Cl. X.R.

